The present invention relates, in general, to respirator apparatus for delivery of breathing gases to a patient, and more particularly to a gas delivery cylinder for providing, at a limited positive pressure, a preselected limited volume of gas at a preselected limited flow rate.
This application is related to copending application Ser. No. 402,677, now U.S. Pat. No 3,905,362, of Theodore B. Eyrick, Allen C. Brown, and Neil R. Hattes, entitled "Volume-Rate Respirator System and Method" filed on even date herewith, and to copending application Ser. No. 402,678, now U.S. Pat. No. 3,887,795, of Theodore B. Eyrick and Neil R. Hattes, entitled "Respiration Ratemeter," filed on even date herewith, the disclosures of which are hereby incorporated herein.
In the treatment of respiratory ailments, in the introduction of anesthetics into the breathing gases of patients, in the control or regulation of a patient's breathing, and in similar and well known uses of respirators or ventilators, it is frequently necessary to provide means for accurately and repeatedly controlling or limiting the volume, flow rate, and pressure of the breathing gases delivered to the patient. Such controls and their accompanying indicators and alarms are necessary in order to avoid not only discomfort, but in many cases outright danger to the patient. Over the years, a great variety of clinical equipment has been developed and employed in the prior art to provide such controls, but the various limitations inherent in the existing devices have created a need for machines of increased reliability, flexibility, capacity, and safety not only to improve the quality of treatment being received by the patient, but to insure his safety against any possible failures of the equipment or its control system.
One of the basic elements in any such respirator system is the unit which receives the breathing gases from a source of supply and which then delivers those gases under pressure through suitable tubing to the patient. Such units have taken a variety of forms in the prior art, with one of the more common units utilizing a mechanical piston to deliver predetermined volumes of air. The piston may be driven, for example, by a variable ratio crank and wheel mechanism connected to an electric motor, but such devices have been found to have certain limitations. For example, such a device is not inherently pressure limiting because of the continuous rotation of the crank and wheel mechanism which serves to mechanically force the piston through a given path on each cycle. Variations in the patient resistance can result in high pressures, and such systems therefore require a pressure relief valve to prevent injury. However, such valves can fail or can with age change in their characteristics, and to this extent are unreliable and unsafe. Additionally, in the devices of this type the maximum rate of flow of the delivered gas is not directly known or controlled and this is a potential source of danger to the patient. Finally, it is very difficult to deliver accurate volumes of gas with such devices because too often the actual rate of delivery is not known.
A common alternative to the piston and cylinder arrangement utilizes a cylinder with an internal bellows which is pneumatically driven by a positive pressure pump. The motion of the bellows is controlled by an external potentiometer which senses the position of the bellows, and by means of a control system which operates solenoid valves to regulate the pneumatic drive lines. Although such systems are satisfactory in many cases, the bellows type device presents some problems since it is difficult to deliver an accurate volume of gas, due to the high compliance of the internal bellows. This compliance allows the position of the bellows to vary from the nominal location, and thus prevents accurate and repeatable delivery of preset volumes of gas. Further, such systems are not inherently pressure limiting, for a failure of the control system during delivery of gas can result in an excessive maximum pressure; accordingly, such systems require a separate pressure relief valve with its attendant shortcomings. The pneumatic drive system normally used with devices of this type presents some difficulty in that such systems are relatively slow to respond to controls, and thus the flow delivery curves are relatively slow. Finally, it is not unusual to find that a substantial portion of the volume of gas to be delivered to the patient remains stored and compressed within the bellows at the end of a delivery cycle, thereby reducing the maximum volume of gas capable of being delivered to the patient.
Other systems of delivering gas to a patient include means for providing a mechanical bias which serves to drive a bellows arrangement. However, such a biasing arrangement presents difficulties in that the flow rate of the system is not limited and it is difficult to deliver accurate volumes of gas due to the high compliance of the bellows and because of variations in characteristics of such devices. Further, in most such devices substantial quantities of gas remain stored within the bellows at the end of a delivery stroke.